1. Field of the Invention
The present invention relates to a grid interconnection device and a grid interconnection system which includes a power conversion circuit for converting power from a power generation device into predetermined alternating-power, and which connects the power generation device to a power distribution system through the power conversion circuit and a relay.
2. Description of the Related Art
Conventionally, there has been known a system, as shown in FIG. 1, in which a plurality of power distribution systems (banks) are arranged under a power distributing substation 20, and in which a large number of customers 100a to 100c are connected in parallel to a bank 1. Specifically, each of the customers 100a to 100c includes a grid interconnection system having a power generation device and a grid interconnection device which connects the power generation device to the power distribution system. In such a grid interconnection system, when power supply to a predetermined customer (for example, the customer 100a) from the power distributing substation 20 is stopped, processing of separating the power generation device in the predetermined customer from the power distribution system is performed. However, in rare cases, disruption of power supply from the power distribution system cannot be detected and thus the separation processing is not performed. The above described state, in which the power supply from the power distributing substation 20 is stopped while the power is supplied by the power generation device in the customer 100a, is called as an islanding state. As a method for detecting the islanding state, there are a passive islanding detection method and an active islanding detection method.
The former method is a method for detecting the islanding state by using a variation in the power distribution system caused when the power supply from the power distribution system is stopped, for example, a phase change (phase jump phenomenon) in a voltage of the power distribution system, a frequency change, a voltage change and the like. Meanwhile, the latter method is a method for detecting the islanding state by applying disturbance, for example, an active variation such as a frequency variation, a phase variation, a voltage variation and a load variation, to an output of the grid interconnection system in the customer, and by using a variation in the power of the power distribution system according to the disturbance. Since the two methods have different characteristics, the interconnection guideline in Japan specifies simultaneous use of the two methods to increase power failure detection accuracy. A power failure can be normally detected in most cases by use of the above methods. However, complete detection cannot be achieved.
In consideration of the circumstances as described above, there has been proposed a transfer trip method for separating a power generation device from a system by transmitting a power failure signal in a power distribution system, which is notified from a power distributing substation, to a grid interconnection system by use of transmission means.
According to one related art, a transfer main device is provided in a power distributing substation and a transfer sub-device is provided in a grid interconnection system in a customer. By using a power line connecting the main device to the sub-device, a carrier is constantly transmitted from the transfer main device and is received by the transfer sub-device. When the carrier no longer reaches the transfer sub-device, a power receiving relay is cut off to prevent an islanding state in the grid interconnection system in the customer.
According to another related art, positions of respective power generation devices and state information of switches in a system are obtained and stored. Based on the stored information, it is individually identified under which distribution lines in which power distributing substations the respective power generation devices are located. When an accident happens in the system, a transfer trip signal is selectively transmitted only to the power generation device which is targeted for transfer trip, based on the above identification. As described above, in the another related art, since a destination is specified and power failure information is transmitted thereto, the transfer trip target can be set to the minimum.